Deconstructing A Simple Op-Amp

Maybe you are familiar with the op-amp as an extremely versatile component, and you know how to quickly construct a huge variety of circuits with one. Maybe you even have a favorite op-amp or two for different applications, covering many possible niches. Standard circuits such as an inverting amplifier are your bread and butter, and the formula gain=-Rf/Ri is tattooed on your forearm.

But you can know how to use op-amps without really knowing how they work. Have you ever peered under the hood of an op-amp to find out what’s going on in there? Would you like to? Let’s take a simple device and examine it, piece by piece.

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Laser Galvo Control via Microcontroller’s DAC

Mirror galvanometers (‘galvos’ for short) are the worky bits in a laser projector; they are capable of twisting a mirror extremely quickly and accurately. With two of them, a laser beam may be steered in X and Y to form patterns. [bdring] had purchased some laser galvos and decided to roll his own control system with the goal of driving the galvos with the DAC (digital to analog) output of a microcontroller. After that, all that was needed to make it draw some shapes was a laser and a 3D printed fixture to hold everything in the right alignment.

The galvos came with drivers to take care of the low-level interfacing, and [bdring]’s job was to make an interface to translate the 0 V – 5 V output range of his microcontroller’s DAC into the 10 V differential range the driver expects. He succeeded, and a brief video of some test patterns is embedded below.

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Taking a Guitar Pedal From Concept Into Production

Starting a new project is fun, and often involves great times spent playing with breadboards and protoboards, and doing whatever it takes to get things working. It can often seem like a huge time investment just getting a project to that functional point. But what if you want to take it to the next level, and take your project from a prototype to a production-ready form? This is the story of how I achieved just that with the Grav-A distortion pedal.

Why build a pedal, anyway?

The author, shown here with bandmates.

A long time ago, I found myself faced with a choice. With graduation looming on the horizon, I needed to decide what I was going to do with my life once my engineering degree was squared away. At the time, the idea of walking straight into a 9-5 wasn’t particularly attractive, and I felt like getting back into a band and playing shows again. However, I worried about the impact an extended break would have on my potential career. It was then that I came up with a solution. I would start my own electronics company, making products for musicians. Continue reading “Taking a Guitar Pedal From Concept Into Production”

Cleaning up a Low-Cost Buck-Boost Supply

Cheap DC-DC converters have been a boon on the hobbyist bench for a while now, but they can wreak havoc with sensitive circuits if you’re not careful. The problem: noise generated by the switch-mode supply buried within them. Is there anything you can do about the noise?

As it turns out, yes there is, and [Shahriar] at The Signal Path walks us through a basic circuit to reduce noise from DC-DC converters. The module under the knife is a popular buck-boost converter with a wide input range, 0-32 VDC output at up to 5 amps, and a fancy controller with an LCD display. But putting the stock $32 supply on a scope reveals tons of harmonics across a 1 MHz band and overall ripple of about 66 mV. But a simple voltage follower built from a power op-amp and a Zener diode does a great job of reducing the spikes and halving the ripple. The circuit is just a prototype and is meant more as a proof of principle and launching point for further development, and as such it’s far from perfect. The main downside is the four-volt offset from the input voltage; there’s also a broad smear of noise at the high end of the spectrum that persists even with the circuit in place. Centered around 900 MHz as it is, we suspect a cell signal of some sort is getting in. 900 kHz.

If you haven’t checked out the videos at The Signal Path, you really should. [Shahriar] really has a knack for explaining advanced topics in RF engineering, and has a bench to die for. We’ve covered quite a few of his projects before, from salvaging a $2700 spectrum analyzer to multiplexing fiber optic transmissions.

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A Very Accurate Current Probe

There’s many different ways of measuring current. If it’s DC, the easiest way is to use a shunt resistor and measure the voltage across it, and for AC you could use a current transformer. But the advent of the Hall-effect sensor has provided us a much better way of measuring currents. Hall sensors offers several advantages over shunts and CT’s – accuracy, linearity, low temperature drift, wider frequency bandwidth, and low insertion loss (burden) being some of them. On the flip side, they usually require a (dual) power supply, an amplification circuit, and the ability to be “zero adjusted” to null output voltage offsets.

[Daniel Mendes] needed to measure some fairly high currents, and borrowed a clip-on style AC-DC current probe to do some initial measurements for his project. Such clip on current probes are usually lower in accuracy and require output DC offset adjustments. To overcome these limitations, he then built himself an invasive hall sensor current probe to obtain better measurement accuracy (Google Translated from Portugese). His device can measure current up to 50 A with a bandwidth stretching from DC to 200 kHz. The heart of his probe is the LAH-50P hall effect current transducer from LEM – which specialises in just such devices. The 25 mV/A signal from the transducer is buffered by an OPA188 op-amp which provides a low output impedance to allow interfacing it with an oscilloscope. The op-amp also adds a x2 gain to provide an output of 50 mV/A. The other critical part of the circuit are the high tolerance shunt resistors connected across the output of the LAH-50P transducer.

The rest of his design is what appears to be a pretty convoluted power supply section. [Daniel] wanted to power his current probe with a 5V input derived from the USB socket on his oscilloscope. This required the use of a 5 V to 24 V boost switching regulator – with two modules being used in parallel to provide the desired output power. A pair of linear regulators then drop down this voltage to +15 / -15 V required for the trasducer and op-amp. His blog post does not have the board layout, but the pictures of the PCB should be enough for someone wanting to build their own version of this current sensor.

You Know You Can Do That with a 555

Hardly a week goes by that we don’t post a project where at least one commenter will lament that the hacker could have just used a 555. [Peter Monta] clearly gets that point of view. For a 555 design contest, he created both digital logic gates and an op amp, all using 555 chips. We can’t quite imagine the post apocalyptic world where the only surviving electronic components are 555 chips, but if that day were to come, [Peter] is your guy.

Using the internal structure of the 555, [Peter] formed a basic logic gate, an inverter, latches, and more. He also composed things like counters and seven-segment decoders. He had a very simple 4-bit CPU design in Verilog that he was going to attempt until he realized it would map into almost 400 chips (half of that if you’d use a dual 555, but still). If you built this successfully, we would probably post it, by the way.  You can see a video of the digital logic counter, below.

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Convert Temperatures the Analog Way

Everyone knows how to convert from Celsius to Fahrenheit, right? On a digital thermometer you just flick the little switch, on a weather app you change the settings, or if worse comes to worse, you let Google do the math for you. But what if you want to solve the problem the old-fashioned way? Then you pull out a few op amps and do your conversions analog style.

We’ve seen before how simple op amp circuits can do basic math, and the equation that [Kerry Wong] wants to solve is even simpler. Recalling the old T= 9/5·Tc + 32 formula (and putting aside the relative merits of metric versus traditional units; we’ve had enough of that argument already), [Kerry] walks us through a simple dual op amp circuit to convert the 1 mV/°C output of a thermocouple module to 1 mV/°F. The scaling is taken care of by a non-inverting amplifier with resistors chosen to provide a gain of 1.8, while the offset is handled by a differential amplifier that adds 32 mV to the scaled input. Strategically placed trimmers allow [Kerry] to tweak the circuit to give just the right conversion.

For jobs like this, it’s tempting to just use an analog input on an Arduino and take care of conversions in code. But it’s nice to know how to do it old school, too, and hats off to [Kerry] for showing us the details.

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